1. Field of the Invention
This invention relates to telescopic rifle sights. More particularly, the invention relates to a telescopic gunsight equipped with a transparent aiming reticle and a method for using that reticle.
2. Description of the Prior Art
It is well known that the line of sight between a rifle shooter""s eye and a target is a straight line, whether using xe2x80x9cironxe2x80x9d sights or a telescopic sight, while the trajectory of the rifle bullet is never a straight line (when passing though a gravitational field, the trajectory follows a ballistic parabola), particularly for bullet flights over long distances. Accordingly, in order to achieve accurate shot placement, it is essential either to xe2x80x9csight inxe2x80x9d the rifle for the target distance or to know both the distance from the rifle to the target and the trajectory characteristics of the bullet used. Trajectory characteristics for commercial bullets are known and are easily obtained from either the manufacturer or from trajectory charts such as the INGALLS"" tables.
Telescopic gunsights, often referred to as xe2x80x9cscopes,xe2x80x9d generally contain a transparent flat disk reticle positioned in a plane perpendicular to the line of sight through the scope. The reticle conventionally has a single vertical crosshair (or hairline) and a single horizontal crosshair (or hairline), which intersects the vertical hairline near the visual center of the reticle. The point of intersection of these crosshairs constitutes the primary sighting point for the scope, representing the site of bullet impact at a chosen (zero) distance. The gunsight is moveable in vertical and horizontal directions by way of calibrated adjustment screws on the exterior of the scope (internal adjustments), or on the scope attachment system (external adjustments).
By firing a series of shots while making compensatory adjustments of the relative position of the gunsight and thus of the reticle center point, the shooting system comprised of rifle, bullet type, scope and shooter are xe2x80x9czeroed inxe2x80x9d so that the position on the target of the crossed hairlines or center point of the reticle coincides with the point of bullet impact.
In certain scope sighting systems the reticle has a series of evenly spaced secondary horizontal hairlines that intersect the vertical hairline below the center horizontal hairline. In those systems, the respective points of intersection of the secondary hairlines with the vertical hairline are typically used to estimate bullet impact points at distances progressively greater than that at which the rifle was xe2x80x9czeroed inxe2x80x9d with the main (center) horizontal crosshair. However, in order to utilize the secondary crosshairs with accurate results, the shooter must know with some precision the distance from gun to target.
Various types of range finder systems have been disclosed for telescopic gunsights. For example, U.S. Pat. No. 1,190,121 to Critchett discloses a reticle having a series of target-spanning rulings disposed above a baseline, the rulings corresponding to associated shooting distances. In use, the shooter ascertains which ruling above the baseline makes the most closely embracing fit on the target, thereby determining the shooting distance (target range). A separate crosshair aiming point is included in the reticle for use in association with each chosen ruling above the baseline.
The principle of the Critchett target-spanning rulings is that certain targets are of known, or at least estimable size. For instance, it is a fairly accurate estimate that for mature deer or antelope, the distance between the top of the back at the shoulders and the bottom of the chest cavity is about 18 inches. The target-spanning rulings are spaced apart such as to span a known target size at a known range. This manner of distance measurement is consistent with conventional trigonometric considerations wherein the triangle defined by the height of the target and the viewing angle through the telescope""s optical system can be considered a right triangle, which accordingly establishes the length of the base line distance to the distal side of the triangle, namely the distance to the target.
U.S. Pat. No. 3,392,450 to Herter et. al. discloses a reticle having a series of target-spanning circles of different diameters which correspond to associated shooting distances. Employing the same basic distance-measuring concept as Critchett, the shooter employs for aiming purposes, that crosshair which corresponds to the selected circle.
U.S. Pat. No. 3,190,003 to O""Brien concerns a range-finding reticle for a telescopic gunsight having single centered vertical and horizontal hairlines. The portion of the vertical hairline below the horizontal centerline is provided with widened bar regions extending various lengths below the centerline. Each bar subtends a target of known size. By finding which widened region corresponds to the height of the target, the shooting distance is estimated.
U.S. Pat. No. 3,431,652 to Leatherwood discloses a telescopic gunsight wherein the distance to the target is determined by movement of upper and lower horizontal hairlines along a fixed vertical hairline in a manner so as to bracket the target. Once bracketed, the intersection of the lower horizontal hairline with the vertical hairline serves as the crosshair aiming point. In this aiming process, the alignment of the scope changes with respect to the gun barrel, whereby the allowance for distance is achieved when the centered crosshair is sighted directly on the target.
U.S. Pat. No. 3,492,733 to Leatherwood discloses a distance measuring system for a variable power telescopic sight that is pivotally moveable in a vertical plane with respect to the gun barrel upon which it is mounted. Cams within the scope and rotatable by external means achieve vertical movement of the scope so that horizontal framing hairlines will fit the target. A specialized cam must be installed into the scope for each particular type of ammunition employed.
U.S. Pat. No. 3,948,587 to Rubbert concerns a variable power telescopic sight having a reticle provided with a vertical hairline, a center horizontal hairline and three horizontal framing lines disposed below the center horizontal hairline. Aiming is achieved by positioning either the center crosshair or lower crosshairs on the target, as dictated by the observed fit of the target within the framing lines.
U.S. Pat. No. 4,403,421 to Shepherd discloses a telescopic gunsight having spaced apart primary and secondary reticles which are moveable relative to each other. The secondary reticle is also moveable vertically and horizontally within the plane of the reticle. The moveable two reticle system facilitates adjustments for windage and elevation. Distance to the target is ascertained by framing indicia on the secondary reticle.
The telescopic sights disclosed in the aforementioned prior art patents are often of limited usefulness insofar as they do not address many of the several factors that need to be considered in the accurate aiming of a rifle under field conditions. Such factors include:
a) distance to target
b) drop of bullet caused by force of gravity
c) hold-over or hold under points
d) wind drift correction
e) correction for phenomenon associated with gyroscopic forces on a gyroscopically stabilized bullet (sometimes referred to as)
1) Yaw of Repose effects (vertical displacements)
2) Magnus effects (horizontal displacements)
These latter result from the effect of cross-wind or shooting either up-hill or down-hill.
Older reticle systems often require that the shooter look away from the target in order to make compensating adjustments. Some of these designs may render the scopes difficult or slow to use, and some require moveable mounting on the rifle, a situation which subjects the scope to inaccuracy after repeated use or abuse in rugged field conditions. Moreover, correct use of any of these systems requires the shooter to manage extraordinary mental work.
The present invention is embodied in a reticle for a gunsight and a precise method for using the reticle to measure distance to any size target, to provide precise drop compensation aiming points for up to approximately 400 yards beyond any high velocity bullet""s normal point-blank (zero) range, to provide precise compensation for cross-winds and up-hill or down-hill shooting conditions, and to provide an accurate lead point for moving targets, thereby providing an accurate and effective method for aiming the rifle, all with easy and fast mental work that does not require extraordinary effort by the shooter or any knowledge of the particular ballistic characteristics of load or gun used.
The telescopic sighting system incorporates an optical system comprised of a forward objective lens element, a rear eyepiece lens element and intervening erector lens element, the elements being protectively confined within an elongated tubular housing adapted to be affixed to a firearm such as a hunting rifle. The improvement provided by the present invention comprises the addition into said optical system within said housing of a transparent reticle having indicia which simultaneously provides accurately both the function of distance measuring and the range specific aiming function. The reticle is positioned between the objective lens element and the erector lens element. The indicia incorporates orthogonally intersecting center vertical and horizontal hairlines, and four horizontal range marker and wind bar lines disposed below the center horizontal hairline with very specific vertical spacings and intersecting in a bisected relation the center vertical hairline.
The specific and precise configuration and positioning of the range marker and wind bar lines enables the shooter to mentally compute the range to the target and allow for bullet drop, drift, gyroscopic effects, up-hill or down-hill angle shots and target lead. With modest practice, a typical shooter can learn to accomplish these tasks within in a split-second. The specific ratio of the spacings of these secondary indicia is critical to the functionality of this system. The accuracy achieved by this reticle promotes shooter confidence which in turn lead to shooter proficiency.